EXPRESSION PATTERN OF AN AXOLOTL FLOOR PLATE-SPECIFIC FORK HEAD GENE REFLECTS EARLY DEVELOPMENTAL DIFFERENCES BETWEEN FROGS AND SALAMANDERS

Gastrulation is one of the most important stages of animal development and, as such, tends io be remarkably conserved. Therefore ii is inter esting to see that the two amphibian species, Xenopus laevis (frog) an d Ambystoma mexicanum (axolotl), are different in the arrangement of c ell types lust before and during gastrulation. in Xenopus, the cells t hat will form dorsal mesoderm are located deep in the dorsal marginal zone, while in the axolott, these are on the surface of the embryo. in this study we investigated whether homologous genes known io be invol ved in the formation of dorsal structures show a different pattern of expression in these two species. For this purpose, we isolated a fork head gene (AxFKHI) from the axolotl, which is likely io be the homolog ue of the Xenopus fork head gene, XFKH1 (Pintallavis, XFD-1). We find that AxFKH1 and XFKH1 have a similar pattern of expression, but there are some important differences. In early gastrulae, transcripts are de tected in the organizer region of both species. In late gastrulae, the transcripts in Xenopus are located in both the superficial and deep l ayers, but they are only found in the superficial layer of axolotl emb ryos. During neurulation, XFKH1 is expressed in notochord and neural f loor plate, whereas AxFKH1 is expressed in the neural floor plate only We propose that the differences in expression pattern of these two ge nes are due io a difference in formation of dorsal structures between these two species. Furthermore, the expression pattern of these two ge nes early in gastrulation is consistent with the idea that al least so me of the neural floor plate cells are already determined at this time . (C) 1997 Wiley-Liss, Inc.